The present invention relates generally to an improved apparatus for mixing fluids and, more particularly, to a mixing apparatus using a noncircular jet of small aspect ratio.
In a wide variety of applications, it is desirable to mix two fluids of the same phase within a relatively small distance. Such applications include, for example, combustion processes, chemical reactions, heat transfer processes, laser technology, environmental control systems and sprayers.
In the past, fluid mixing has often involved bringing a circular jet of one fluid into contact with a second fluid of the same phase. Mixing is accomplished by spreading of the jet. A naturally unstable flow condition at the boundary of the jet forms a vortex which causes the jet to spread and entrain the second fluid. The vortex increases rather slowly in size in the downstream direction and defines a uniform shear layer around the potential core of the jet.
Another form of turbulent jet which has been used and studied in the past is a two-dimensional or "planar" jet which is long and narrow enough in cross section that its end effects are negligible. When the "major axis" of a jet is defined as the direction of its maximum lateral dimension, and the "minor axis" is perpendicular to the major axis, the planar jets of the prior art have a major axis dimension at least five times as great as the minor axis dimension. In some cases, the major axis dimension may be as great as 20 or 40 times the minor axis dimension. Such two-dimensional jets produce a uniform shear layer similar in concept to the shear layer of a circular jet, and tend to grow rather slowly in the downstream direction. This limits entrainment of the second fluid.
Circular and two-dimensional jets have been studied, as described in Wygnanski, I., AERO. QUART. 15, 373 (1964), and Ho and Hsiao, 1982 Proceedings of IUTAM Symposium on Structure of Complex Flow, Marseille, France, Springer-Verlag. The results of such studies are depicted in FIG. 8, where the curve 84 and the dashed line represent entrainment achieved with an axisymmetric (i.e., circular) jet, and the square markings represent entrainment achieved with two-dimensional jets. The two-dimensional jet used by Ho and Hsiao in obtaining these results had an aspect ratio (i.e., ratio of the major axis dimension to the minor axis dimension) of 24 to 1. As shown in FIG. 8, the mass entrainment achieved with two-dimensional jets is approximately the same as that achieved with simple axisymmetric jets.
Attempts have been made to enhance fluid mixing by distorting the edge of an axisymmetric orifice from which a jet is emitted. For this purpose, portions of the edge are bent alternately in and out to form a series of tabs which generate small disturbances or perturbations in the flow. The disturbances give rise to small eddies and enhanced mixing. However, the enhancement achieved in this way has been limited to between 5 and 15 percent.
Another known method of enhancing fluid mixing is to externally "force" a fluid flow, as described in J. Fluid Mech., 119, 443 (1982). In the context of a jet, external forcing involves pulsation of the velocity or pressure of the jet to increase entrainment. For example, a sinusoidal pressure variation can be applied to the jet for this purpose. Although external forcing increases mixing efficiency, it also consumes external energy and is inappropriate in a large number of applications.
Elliptic vortex rings have been studied in contexts other than fluid mixing, as discussed in Dhanak and De Bernardinis, J. Fluid Mech., 109, 189 (1981). Vortex rings are transitory flows in the nature of "puffs", and have little in common with fluid jets of the type used in mixing. Whereas a jet is a constant flow of fluid, there is no mean flow in a vortex ring. Vortex rings have been studied recently as an aid in understanding the nature of vortices generated at the tips of airplane wings. In the course of such studies, it was found that elliptical vortex rings undergo repeated transitions after they are formed, by which the major and minor axes of the rings switch back and forth. To the best of applicant's knowledge, this phenomenon, known as "vortex induction", has not been considered applicable in any way to fluid mixing, nor has it been suggested that the phenomenon would occur under constant flow conditions.
Finally, nonaxisymmetric jets of small aspect ratio are discussed in Hayes U.S. Pat. No. 3,201,049, in connection with an aspirating garden hose sprayer for applying liquid chemicals to plants. A primary fluid, water, is introduced to an aspirating chamber through an inlet passage which may be square, rectangular or triangular in cross section. The specification teaches that the inlet passage should have at least two straight sides for maximizing turbulence within the chamber. However, the geometry of the aspirating chamber and subsequent passages are such that the jet emitted by the inlet passage is confined in the lateral direction once it travels a short distance from the inlet passage. In the embodiment of FIG. 1, the downstream end of the inlet passage also diverges at an acute angle which would cause separation of the flow from the orifice wall. Thus, the Hayes device is designed to accurately proportion a relatively small amount of a liquid chemical into a water stream, but is not suited to entraining large quantities of the liquid at high efficiency.
Therefore, it is desirable in many applications to provide an apparatus for efficiently mixing two fluids in a relatively short distance and without the need for external energy.